Process of drying wood by oil immersion and vacuum treatment to selected moisture content with oil recovery



Sept. 14, 1965 3,205,589

PROCESS oF DRYING wooD BY OIL IMMERSION AND VACUUM TREATMENT R. D. FIESETAL TO SELECTED MOISTURE CONTENT WITH OIL RECOVERY Filed Sept. 27. 19612 Sheets-Sheet 1 bbw MES: e

w M m Sept. 14, 1965 R. D. FlEs ETAL 3,205,589

PROCESS OF DRYING WOOD BY OIL IMMERSION AND VACUUM TREATMENT TO SELECTEDMOISTURE CONTENT WITH OIL RECOVERY Filed sept. 2v, 1961 2 sheets-sheet 2United States Patent O 3,205,589 PROCESS OF DRYIN G WOOD BY OILIMMERSEON AND VACUUM TREATMENT TO SELECTED MOISTURE CONTENT WlTH OiLRECOVERY Richard l). Fics and William W. Johnston, Bend, Oreg.,assignors to West-Wood Processing Corporation, Bend,

Greg., a corporation of Nevada Filed Sept. 27, 1961, Ser. No. 141,054Claims. (Cl. :i4-9.5)

The present invention relates to improvements in drying of wood productsby immersion in hot oily liquid under controlled conditions oftemperature, pressure and circulation so that a low moisture content issubstantially uniformly established throughout the Wood regardless ofvariations in initial moisture content and variations in nature of thewood liber. In addition, important aspects of the invention involveeffective recovery of the drying oil from the Wood without materialdisturbance of the moisture content of the wood.

Techniques are known for rapidly drying wood by a socalledboiling-in-oil treatment, as discussed for example in Report No. 1665(Revised) entitled Special Methods of Seasoning Wood; Boiling in OilyLiquids, dated April 1956, and published by the Forest ProductsLaboratory (FPL), an agency of the U.S. Dept. of Agriculture ForestService. However, previous attempts to utilize the rapid dryingadvantage of such oil drying techniques have not enjoyed commercialsuccess in terms of being competitive with established wood dryingtechniques. This failure has apparently been due to an inability tocontrol the final moisture coutent of the wood, an inability to recoversuflicient oil to avoid an oily appearance in the wood, and because ofthe substantial added cost of the wood-retained oil, as well as itsadverse effect on the flammability of the wood.

A drying oil having a higher boiling point than water cannot be removedfrom wood to the degree commercially necessary in any reasonable lengthof time simply by forced air circulation. To get a more rapid recoveryof the oil, attempts have been made to increase the temperature at whichthe residual oil is evaporated or vaporized from the wood, but suchincrease in temperature to about the boiling point of the oil provesinjurious to the properties of the wood in terms of surface cracking,charring, and case hardening. Further, and quite importantly withrespect to many commercial usages of wood, a higher order of dryingtemperature which is well above the boiling point of water entailsconsiderable further reduction in the moisture content of the wood evento the point of desiccation.

For some usages, certain forms of wood products such as veneer shouldnot be dried beyond a low but substantial moisture content, in order tohave good bonding or gluing properties. Also, with respect to Woodproducts such as veneer which are bonded or glued by a so-called hotpress process, any substantial retention of oily liquid in the woodgives rise to an explosion hazard, so is commercially unacceptable.Further, with respect to air scavenging of drying oil from the wood,this procedure of itself has proven sometimes to generate serioushazards because oil vapor and air mixtures can be explosive.

Other attempts have been made to recover drying oil from oil dried woodby vacuumizing the chamber containing the wood. However, vacuumizingtechniques have proven unsatisfactory in that while the relative partialpressure of the oil is somewhat improved, the lack of environmentcirculation results in a poor recovery rate. Vacuumizing alone also hasan adverse effect on the moisture content of the Wood, since therelative partial pressure of the contained moisture is also improved andthe wood continues to dry. It has also been found that if ICC there isany substantial evaporation of moisture from the wood during residualoil recovery, such moisture evaporation results in rapid loss ofresidual heat from the wood, which tends to dampen the evaporationrecovery of the oil before a practical amount of oil is removed from thewood.

Steam, when used as a scrubbing agent for the drying oil at atmosphericand superatmospheric pressures, does not provide sufficiently rapidrecovery rates to be commercial, because of the relatively low partialpressure of the oil at atmospheric and superatmospheric pressures.

For a more detailed consideration of various prior techniques having todo with oil drying of wood and the attendant dificulties involved,further reference should be made to the aforesaid FPL Report 1665(Revised); to the original FPL Report R1665, entitled Special Methods ofSeasoning Wood; Boiling in Oil, dated February 1947; to FPL Report1665-1 (Revised), entitled Special Methods of Seasoning Wood; HighTemperature Drying: Its Application To The Drying Of Lumber, datedNovember 1957; to FPL Report 1665-2 (Revised), entitled Special Methodsof Seasoning Wood; Solvent Seasoning, dated March 1955; to FPL ReportR1665-5 entitled Special Methods of Seasoning Wood; Vacuum Drying ofWood, dated May 1953; to Western Pine Association Research Note No.4.311, entitled Seasonwood Process, dated May 15, 3; to FPL Report 1642,entitled How Wood Dries, dated September 1956; to Hutchinson U.S. Patent2,892,261, and to McDonald U.S. Patent 2,860,070.

One prior oily liquid type wood drying procedure, as disclosed in saidMcDonald Patent 2,860,070, involves use of a halogenated hydrocarbonsuch as perchloroethylene, wherein evaporative extraction of the liquidis by removal of an azeotropic mixture of the water and liquid, underconditions including sequenced vacuumizing and steaming. However,removal of the drying liquid as an azeotropic mixture has thedisadvantage of removing further moisture as well as drying liquid fromthe wood, with the result that no effective control as to uniformity offinal moisture content can be realized. In fact, an optional procedurewhen using the McDonald process is that of periodically steaming thewood, apparently to restore a sufficient moisture content therein topermit formation of the characteristic azeotropic mixture.

In contrast to such prior oil drying and oil recovery procedures, theprocessing technique characteristic of the present invention has thecapability of permitting close control of moisture content of the wood,both in the drying operation and in effecting an efficient recovery ofdrying oil from the wood. Notwithstanding its capability of drying woodto a controlled and readily predeterminable moisture content, theprocessing of the present invention has the capability withincommercially practical processing times of recovering the higher boilingdrying oil to the extent of leaving in the wood less than about 0.3%oil, based on the oven dry weight of the wood, so as to avoid anypossibility of an explosion hazard in certain wood product fabricationprocesses to Which the wood may be subjected. In contrast, it is to berecognized that oil removal by forced air circulation or by steam atatmospheric pressure has proven to leave on the order of 2% or more oilin the wood. In addition, the drying 'and oil extraction treatment ofthe present invention involves temperature conditions within the woodwhich are not so severe as to cause surface deterioration and strengthloss in the wood.

One important aspect of the invention is its realization of fasttreatment times, both as to the oil drying step and as to the oilrecovery step, so that Wood drying can be integrated with Woodutilization operations involving fast throughput rates. For example, inthe case of veneer for utilization in the fabrication of plywood, acontinuous or substantially continuous drying operation according t thepresent invention can be integrated with a continuous peeling operation,avoiding any necessity for clipping of the veneer into separate woodtypes as it comes off the peeling lathe, and obviating the need forseparate drying of the veneer types. Conventional operating procedurenow involves clipping of the freshly peeled veneer according to the typeof wood, i.e. heartwood and sapwood, then hand sorting to segregate thetypes of Wood preliminary to drying. This is because sapwood takessubstantially longer to dry than heartwood when dried by conventionaltechniques. Subsequent to drying, further sorting as to grade must beeffected, often with reclipping. The reason such segregation as to woodtype is necessary in conventional practice is that heartwood dried on asapwood schedule will not effectively bond and y sapwood dried on aheartwood schedule will present wet spots which risk blow out on a hotpress. By virtue of the present invention, the heartwood and sapwood canbe dried together, as in a continuous sheet, and the resulting moisturecontent of each type of wood is exceedingly uniform, and controllablyso, within a practical operating tolerance of less than about i0.5% ofthe desired moisture content.

In practice of the present invention, the residual heat in the wood leftfrom oil drying thereof is utilized to provide the desired temperaturefor oil recovery, and the evaporation of incident moisture from the woodis inhibited so that the wood does not cool too rapidly. In thisconnection, it is to be particularly observed that the superheated steamwe employ for drying oil recovery is not primarily a heat source, butrather a moisture stabilizer and scrubbing or stripping agent forremoving oil vapor as it diffuses to the surface of the wood. Viewed inanother manner, the slow dropotf of residual heat level of the Woodduring oil recovery according to the present invention is considered toresult primarily from the moisture stabilization provided by thesuperheated steam, since such moisture stabilization prevents loss ofresidual heat in the form of latent heat of vaporization of the woodcontained moisture.

In terms of the controlled conditions maintained during the oil dryingand oil recovery phases of the process, the functional aspects involvedcan be summarized as follows. During the oil drying phase with an oilhaving a distillation range substantially above the boiling point ofwater, the desired temperature condition is selected to be substantiallyabove the boiling point of water but below the boiling point of the oiland low enough to avoid damage to the wood. With respect to pressure,the oil bath drying is preferably but not necessarily conducted atatmospheric pressure.

A phenomenon of wood drying in a hot oil bath is the formation of alayer of superheated steam bubbles on all surfaces of the wood caused bythe diffusion of the woodcontai-ned moisture in vapor form to thesurfaceY of the wood. This layer of bubbles clings to the surface of thewood until disturbed or until individual bubbles become large enough toovercome the adhesion and surface tension forces which hold them to thesurface of the Wood. Such layer of superheated steam bubbles, in effect,envelopes the wood in an atmosphere of superheated steam. This layeralso, in effect, constitutes a barrier to heat transfer between the oilbath and the wood which retards heat transfer from the oil to the woodin proportion to the thickness of the layer, and can if properlyutilized prevent overdrying and overheating of the wood.

Wood, in an atmosphere of superheated steam, such as f that provided bythe self-generated layer of superheated steam bubbles referred to above,has been found to reach a point of moisture stabilization wherein itneither gains nor loses moisture, i.e. is at equilibrium. At this pontof stabilization, with the moisture content of the wood in equilibriumwith the surrounding layer or blanket of superheated steam atmosphere,the amount of moisture in the wood is essentially dependent upon thetemperature of the superheated steam, substantially according to therelation shown in FIG. 2.

This self-generated, protective drying atmosphere, when properlycontrolled, permits rapid drying of the wood, yet prevents overdryingand overheating thereof, and thereby avoids the harsh effects on thewood which are often considered to be inherent in subjecting wood to ahot oil bath.

The degree to which the layer of superheated steam bubbles shields theWood from the heat of the bath is regulatable to a considerable extentsimply by means of varying the extent of circulation of the oil bath.Characteristic of the present invention, the oil bath is circulatedrapidly for an initial period, then bath circulation is progressivelyreduced during further and final drying to allow the vapor barriersurrounding the Wood to be retained at all times and to maintain thedesired final moisture content in the wood while Wet spots which havenot yet reached equilibrium continue to dry. In contrast to our controlof circulation to achieve a preselected moisture content, it hasheretofore been considered that oil bath drying rof wood should seek todry the wood as much as possible, .as by maintaining strong circulationthroughout the drying phase, without concern as to any preciseuniformity of final moisture content.

Wood in :an atmosphere of superheated steam reaches an equilibriummoisture content dependent upon the temperature of the superheatedsteam, as pointed out above. The temperature of the superheated steamenvelope or blanket is controlled in our process by the temperature ofthe drying oil. The thickness and effectiveness of this envelope orblanket of superheated steam is controlled by regulation of thecirculation rate of the drying oil bath. Strong circulation and sharpimpingement of the drying oil on the steam envelope produces a scrubbingaction which partially overcomes the adhesion forces holding theenvelope of superheated steam to the surface of the wood, whichdiminishes the barrier presented to heat transfer from the drying oil tothe wood. Conversely, reduced circulation and reduced impingement of thedrying oil on the surface of the wood by reduction in its circulationrate reduces the scrubbing action and allows a more effective envelopeof superheated steam to surround each piece of Wood, slowing the heattransfer from the drying oil to the wood.

As any particular portion of the Wood is reduced to a moisture contentcommensurate with the equilibrium moisture content determined :by thetemperature of the superheate-d steam blanket, the moisture content ofthe wood will stabilize as long as the vapor blanket is not destroyed.yUnder such conditions, the ibath and the wood are a-t essentially thesame temperature, and the vapor pressure yof Ithe vapor blanketsubstantially equals the hydrostatic pressure and surface tension of theoil. Continued strong circulation of the lbath prevents temperaturerelated (i.e. superheated steam controlled) stabilization of the finalmoisture content of the wood, because the strong circulation continuesto scrub away the vapor blanket. However, if the fbath circulation issufficiently reduced Ibefore the moisture content of the wood is reducedto the moisture content of the superheated steam barrier, t-hen thedrying action essentially ceases at the desired point since the vaporblanket continues to surround the lwood and there is no force present toimpel further movement of water Vapor from the wood. As will beunderstood, relatively wet spots which remain at the time thecirculation is reduced to the point of not interfering with the vaporenvelope will continue to dry in that the temperature of the Wet spotsis still below the temlperature of the bath so the wet spots continue toa'bsonb heat from the 'bath and continue to yield water vapor. -lnessence, the initia'lly relatively strong circulation characteristic ofthe present invention serves primarily to reduce drying time, while thefinal circulation rate is critically established to and serves primarilyto allow the existence of a substantial vapor envelo'pe surrounding theWood during the `final drying.

With respect to condition control during recovery of drying oil from theWood, the desired temperature condition of having the Wood atsubstantially above the boiling point of water is most suitably andpractically established by utilizing the residual heat remaining in thewood from the drying operation. As to pressure condition, it has beenfound that a vacuumized environment is essential for practical oilrecovery rates at temperatures materially below the boiling point of theoil, and in this regard practice has indicated that the pressure shouldbe subat-mospheric to the extent of about one-half atmosphere or less.Coupled with such reduced pressure and elevated temperature conditions,markedly improved oil recovery is effected 4by .adding to theenvironment superheated steam at a temperature having substantially thesame equilibrium moisture content as the desired moisture content of thewood.

Thus, it is seen that the present invention involves a-n oil dryingphase and an .oil recovery phase wherein the wood is controlled as touniformity of moisture content by the maintenance of a superheated steamenvironment which surrounds the wood throughout both phases oftreatment, with such environment being provided by a self-generatingsteam envelope or blanket surrounding the wood througout the dryingphase, and being provided by a continuous impingement of superheatedsteam on the wood during the oil recovery phase under conditions ofreduced pressure, to improve the rate and extent of oil oil recovery.

These and other objects, features, ipurposes and advantages of theinvention vvill be apparent from the following desoription of certainsystem forms and operating procedures characteristic of the invention,taken together with the accompanying illustrations, wherein likenumerals refer to like parts, wherein prime numerals refer tocorresponding parts of similar function, and wherein:

FIG. 1 is a schematic flow diagram of a batch-type oil drying .and oilrecovery system, such as for experimental practice of the invention;

FIG. 2 is a graphical presentation showing the equiliblriurn moisturecontent of Wood in superheated steam environment at varioustemperatures; and

|FIG. 3 is a diagrammatic view of a continuous oil drying and oilrecovery process for treating freshly peeled veneer.

FIG. 1 illustrates a typical, batch-type processing systemcharacteristic of the invention, wherein wood to be dried is placed inseveral separated layers on platform of treating chamber .12, and helddown therein by suitable means (not shown). With the wood in place,t-reating chamber 12 is closed and lled with a drying oil suchas`Standard Odorless Thinner to above the level of lche lumber bydelivering oil from oil storage reservoir 14 through valve 16,circulating pump 18, oil heater 20, flow Iregulation valve 22 andperforated circulation headers '24 arranged along one side of treatingchamber 12 and extending at least most of the length thereof to directsprays or jets of the incoming oil on substantially all exposed woodsurfaces. During the time treating chamber 12 is being tflled, returnvalve 26 is closed. ySteam supply valve 28 remains closed during thedrying operation. Outlet valve 30 in the vapor line 32 leading fromtreating chamber 12 is normally open, and is used only in the event -asuperatmospheric pressure is desired in -chamber 12 during drying, inwhich case it is regulated -to provide constricted outow from thechamber 12, the pressure head in chamber 12 being suitably indicated, asby pressure gage y34.

i lWhen the desired oil level in treating chamber "12 is attained, whichca-n be ascertained by suitable means such as by simply viewing the'liquid Ilevel 36 in viewing window 38, oil storage supply valve I16 isclosed and outflow valve 26 is opened to establish return ow fromchamber 12 to pump 18, as indicated at 40. Then, the desired circulationrate of the oil bath in treating chamber 12 is established Ibyregulation of valve 22, the desired rate of flow ,and correspondinglythe extent of circulation of oil in chamber 12 being suitably indicated,as by owmeter 42.

Assuming an atmospheric pressure condition in treating chamber 12, andassuming the wood being dried is Ma Douglas fir veneer, for example, theoil being recirculated in chamber 12 can suitably :be at a temperatureaveraging about 240 F.290 F., say 265 F., to give a stabilized moisturecontent .of about 3% (cf. FIG. 2). The average bath temperature has:been found to be about 1020 F. less than the incoming oihtemperaturereected by `temperature probe y43, so that the incoming oil temperaturecorresponding to an average temperature of 265 F. is about 280 F., forprocess control purposes.

The drying time involved can be about four minutes, exclusive of `afilling time of about forty seconds, and exclusive of a drain time ofabout one minute. When most of the so-called free water has beenvaporized from the wood (in contrast to the so-called bound Water, i.e.the wood fiber saturating water), the reduced vapor outflow in line 32results in a temperature reduction being rellected by temperature gage44. Concurrently with such temperature reduction, valve 22 isprogressively throttled to reduce the rate of circulation of the oilbath. Such control of bath circulation can be accomplished manually, orcan be effected by temperature responsive flow control means, in amanner known per se. When there is substantially no further water vaporbeing given otf the wood, the temperature reilected by gage 44stabilizes, at which time the drying operation can be discontinuedsimply by opening valve 16, stopping pump 18 and draining the oil backto oil storage 14 through valves 26 and 16, as indicated at 46.

During the oil drying phase, the vapors produced are an admixture of oilvapor and Water vapor (predominantly oil vapor in the case of use ofStandard Odorless Thinner as the oil), and the drying oil vaporizedduring drying is recovered by passage of the vapors through water cooledcondenser 48, with discharge of condensed liquid through condenserdischarge line 50 and valve 52, which is open in the direction indicatedat 54, to separator 56 where the oily condensate and water condensateseparate into two layers `by virtue of relative immiscibility anddifierence in specic gravity. In separator 56, the oily condensate formson the top, and the respective oil and water levels 60 and 62 aresuitably sensed, as by display through a viewing window 64, with theliquid levels being maintained so that the oily condensate overflowsinto return line 66, as indicated at 68. One procedure by which suchdischarge of recovered oil from separator 56 can be effected is by meansof an electrical probe (not shown) or other liquid level indicatingdevice, which senses Water level 62 and controls the extent of outiiowof Water through separator discharge valve 70.

Oil return line 66 from separator 56 returns the oil to chamberdischarge line 72 below valve 26, so that when treating chamber 12 isfilled with recirculating drying oil the level 36 thereof is maintainedat all times lsince oil outflow from separator 56 accumulates with therecirculating oil coursing pump 18. Gravity return separator 56 isadvantageously positioned so that the oil level 60 therein is slightlyabove the oil level 36 in treating chamber 12, as shown.

To provide vapor balance throughout the system, separator 56 and oilstorage 14 are respectively suitably vented as indicated at 72 and 74,and such vents 72, 74 can advantageously be interconnected, if desired.

After the oil bath has been withdrawn from treating chamber 12 at theend of the drying phase, and to initiate the oil recovery phase of theprocess, vacuum pump 76 is started and valve 52 is shifted to placevacuum pump 76 on outflow line 50 from condenser 48, with flow asdesignated at 78, valves 22 and 26 being meanwhile fully closed andvalve 30 fully opened if partially closed during the drying step.

Vacuum pump 76 establishes a subatmospheric pressure in treating chamber12, the extent of partial pressure being suitably indicated as by vacuumgage 80 in vapor line 32. When the partial pressure is suitablyestablished in treating chamber 12, as at a reading of about 12-14inches Hg, a slight amount of superheated steam flow and circulation intreating chamber 12 is established by partially opening steam valve 28which is connected to a suitable steam supply 82 from which steam isavailable at a suitable temperature. For example, the steam delivered tosteam inlet valve 28 can be at a temperature when released into chamber12 which corresponds to the desired equilibrium moisture content in thetreated wood. Should the desired moisture content of the wood be about3% (corresponding to the moisture content established by the averagebath temperature at the wood surfaces of about 265 F. during the dryingphase), the desired steam temperature at temperature gage 43 shouldsubstantially correspond, according to the relation shown at FIG. 2,i.e. be about 265 F., in the example selected. In practice,comparatively little steam vapor provides an adequate irnpingeinent andcirculation to effectively strip the Vaporizing residual oil from thewood surface. The amount of superheated steam vapor introduced to theoil recovery environment suitably can about equal the amount of oilvapor evaporated from the wood. In other words, during the oil recoveryphase, owmeter comparisons can be made of the quantity of watercondensate and the quantity of oil condensate appearing as outflows fromseparator 56, and steam valve 28 regulated to keep these quantities-substantially equal. A comparatively greater amount of steam can beused, but is not to be preferred, since it makes little additionalcontribution to the desired moisture stabilization and scrubbing action.At the temperatures involved, there is no substantial heat transferbetween the steam and wood.

For the earlier indicated example involving drying treatment of lsDouglas fir veneer (half heartwood and half sapwood), oil recoveryutilizing the residual heat remaining in the wood from the dryingoperation and establishing a superheated steam environment in chamber 12for four minutes resulted in a reduction of the residual oil content toabout 0.25%, without substantial change of moisture content, the finalmoisture content being about 3.0%.

Other examples and a comparison of the oil recovery eiiiciency of thesuperheated steam in a vacuumized chamber, and comparisons with resultsobtained by use of superheated steam at atmospheric pressure, and by useof a vacuumized chamber without superheatedsteam, are presented below.

Analyses of selected wood samples taken after the drying phase butbefore the oil recovery phase indicate that the oil content of the woodafter oil drying but before oil recovery varies considerably dependingupon the type of wood and is in general in the range of from about 5% toabout 12%.

As a rst comparative example, showing typical oil recovery underconditions involving steam atmosphere at atmospheric pressure, a sampleof Douglas fir sapwood veneer .220 inch thick with an initial moisturecontent of 65% was dried at an average temperature of about 260 F. foriive minutes according to the procedure discussed in connection withFIG. l, then during the oil recovery phase was subjected to superheatedsteam at about 260 F. at atmospheric pressure for ten minutes. Six inchsections cut from the ends of the veneer after drying and oil recoveryexhibited a final moisture content of 3.4% and a final oil content of1.97%. A corresponding section taken from the center of the veneerexhibited a final moisture content of 3.42% and a inal oil content of1.78%. This example demonstrates that while the resulting moisturecontent was substantially uniform throughout the wood, the oil recoveryby superheated steaming at atmospheric pressure is commerciallyunsatisfactory, leaving a residual of about 2%.

As a second comparative example, showing the effect of using vacuumizingalone for oil recovery, a .200 inch thick Douglas tir sapwood veneerwith an initial moisture content of 12% was oil dried for five minutesat an average temperature of about 265 F., then subjected to a partialpressure of fourteen inches Hg for seven minutes during the oil recoveryphase. The nal moisture content of the veneer was 2.98% and the iinaloil content was 2.2%.

Concerning the extent of time necessary to recover the residual oileffectively, a further comparative example involved .130 inch thickDouglas r sapwood veneer with 18% initial moisture content, which wasoil dried at an average temperature of about 270 F. for four minutes,then subjected to superheated steam at about 275 F. and under a partialpressure of fourteen inches Hg with steam introduction for the period oftwo minutes. The linal moisture content was 2.32% and the final oilcontent was 2.56%. However, a like sample, having an initial moisturecontent of 25% was subjected to a closely similar drying procedure (at278 F.), then vacuumized to fourteen inches Hg, and superheated steamintroduced for four minutes during the oil recovery phase, the resultingsample exhibiting a final moisture content of 2.04% and a final oilcontent of 0.26%. The sharp drop in iinal oil content evinced by thislatter example indicates the oil recovery phase must be carried for asubstantial, although still comparatively short time.

As a further comparative example characteristic of the invention, threepieces of Douglas iir sapwood veneer .130 inch thick with an initialmoisture content of 28% were stacked in the treating chamber Withspacers in between the individual pieces, then subjected for fourminutes to an average drying temperature of about 275 F., thenvacuumized at fourteen inches Hg and steam was trickled in at atemperature of about 275 F. for four minutes. The top piece exhibited aiinal moisture content of 2.62% and an oil content of 0.22%, the middlepiece exhibited a final moisture content of 2.41% and'a final oilcontent of 0.28%, and the bottom piece exhibited a final moisturecontent of 2.31% and a final oil content of 0.21%. The variation inmoisture content in this example as to a comparatively lesser moisturecontent toward the bottom of the chamber was caused, it is believed, bya steam heating coil at the bottom of the treating chamber, which heatedthe lower layers of Wood by radiation during the oil recovery phase, andresulted in relatively higher wood temperatures in the lower layers.

A further example is presented below which demonstrates the uniformityof drying accomplished by the processing of the present invention asbetween heartwood portions of sapwood portions of the wood, and ascompared with thev moisture content levels attained in these varioustypes of Wood by conventional veneer drying techniques involving a hotair environment. The example also serves to show a performanceevaluation as to gluing characteristics, appearance and workability.

To evaluate the properties of veneer oil dried according to the presentinvention, like samples of heartwood and sapwood Douglas fir veneerswere cut from the same pieces, with one sample of each being subjectedto oil drying and oil recovery according to the present invention andwith another sample of each being dried by forced circulation of anenvironment of heated air in a conventional steam heated veneer dryer.The veneer oil dried with vacuumized steaming according to the presentinvention, demonstrated a sapwood moisture content of 2.8% and aheartwood` moisture content of 3.0%, while the conventionally driedsapwood demonstrated a moisture content of 3.6% and the conventionallydried heartwood demonstrated a moisture content of 4.5%. Observations 9.as to the oil dried veneer indicated it had a nearly undetectable odorbefore pressing, and that the odor was not apparent during and after hotpressing the veneer into plywood. As to sanding characteristics, afterpressing, no discernible differences in the oil dried and theconventionally dried plywood panels were observed. As to gluingcharacteristics, glue flow and penetration properties appeared the samein both types of samples, and knifed bonds were comparatively extremelysimilar. Glue spreading characteristics and strength properties werelikewise remarkably similar.

FIG. 2 is a graphical presentation of the approximate equilibriummoisture content of wood in a superheated steam atmosphere, to show thecorrelation involved as between processing temperature and resultingmoisture content in the wood.

PIG. 3 is a diagrammatic view of a modified form of system forpracticing the oil drying and oil recovery techniques of the invention,which system is adapted for use in continuously processing veneer orlike wood while being conveyed in the form of a strip or sheet. Exceptas otherwise indicated, the system components shown in FIG. 3 arefunctionally like those in the system illustrated at FIG. 1, andrespectively corresponding components are designated by prime numerals,while separate components of related function are designated bycorresponding lettermodified numerals.

In the system shown at FIG. 3, the veneer or like wood sheet 100 movessubstantially continuously through drying chamber or tunnel 12A, thencethrough an oil recovery chamber or tunnel 12B, and can receive theveneer 100 directly from the peeler lathe as an integrated system, withsubsequent delivery to a clipper or the like.

Drying chamber 12A contains the drying oil bath, the level of which isindicated at 36'. The veneer 100 is fed into chamber 12A through infeedrolls 102, 104 which comprise a part 'of an ante-chamber 106 wherein ismaintained a steam atmosphere at a pressure slightly above the pressurein the drying chamber, the steam atmosphere in ante-chamber 106 beingprovided from a suitable steam source, as indicated at 108. Chamber 106and rolls 102, 104 provide an essentially vapor tight ingress of sheet100 into drying chamber 12A so substantially no loss of vapors therefromresults. To this end, chamber 106 vents suitably into drying chamber12A, as by flap valve 110.

After entry into chamber 12A, Veneer sheet 100 is submerged below level36 of the oil bath by coursing a series of guide rolls 112, then passesout of the drying chamber 12A through feed rolls 114. Seal flaps 116 areprovided in conjunction with rolls 114 to maintain the drying chamber12A vapor seal. The veneer sheet 100 then proceeds through slottedstructural member 118 and inieed rolls 120 with associated seal iiaps121 into oil recovery chamber 12B, then through a series of guide rolls122, then out of the oil recovery chamber 12B through pairs of vaporseal rolls 124, 126, which comprise a part of exit chamber 12S,providing a vapor seal for oil recovery chamber 12B by maintaining asteam atmosphere at a slightly superatmospheric pressure, such steamatmosphere being suitably supplied, as indicated at 130, the slightvapor outflow from exit chamber 128 being vented into chamber 12Bthrough ap valve 131.

The oil bath in drying chamber 12A is recirculated by pump 18', throughthermostatically controlled heater 20', and is delivered into chamber12A through a series of spray heads 132 arranged above and below theveneer sheet 100, which spray heads 132 serve to provide the controlledcirculation of the oil bath in a manner characteristic of the invention.The extent of the circulation of agitation provided in the oil bath isregulatable by flow control valve 22', and the spacing between thesuccessive spray heads 132 progressively increases along the line oftravel of the veneer sheet 100 through the chamber 12A, as shown, inorder that the veneer sheet 100 be first subjected to strong bathcirculation then to progressively reduced bath circulation. Vaporoutflow from drying chamber 12A is taken ott by outflow line 32a,leading to dryer condenser 48A, thence to separator 56' from whence theoil condensate is returned to circulation through line 66', and thewater condensate is regulatably discharged through valve 70. In vaporoutflow line 32A, a vented pressure relief valve 134 serves as anoverpressure relief valve,

The flow line 136 from the condensate side of drying condenser 48A leadsto vacuum pump 138 and pressure regulation valve 140, either of whichcan feed line 142 leading to separator 56. If a substantiallyatmospheric pressure is desired in drying chamber 12A, vacuum pump 138is not operated, its ow control valve 144 is closed, and pressureregulating valve 140 is fully opened. If a superatmospheric pressure isdesired in drying chamber 12A, pressure regulation valve 140 ispartially closed the degree necessary to reflect the desired pressure onpressure-vacuum gage 146. If a subatmospheric pressure is desired indrying chamber 12A, vacuum pump valve 144 is opened, vacuum pump 138 isoperated, and pressure regulation valve 140 is closed. Regulation of thevacuumization to maintain a desired subatmospheric pressure, asreilected by pressure-vacuum gage 146, is accomplished by valve 144.

Automatic discharge of oil condensate from separator S6 is effected bypump 143, which can -be suitably operated, as by a float Switchmechanism (not shown) sensing the oil level in separator 56.

To start up the drying chamber 12A, and assuming all of the oil isinitially in oil storage 14', valves 22 and 26' are checked to beclosed, valves 16 and 148 are opened, and pump 18 is started torecirculate all of the oil through oil heater 20', line 150 and oilstorage 14. Then, when the oil has attained temperature, valve 22 isopened and valve 148 closed to deliver oil to the drying chamber 12A.When the desired oil level 36 is reached, valve 26 is opened and valve16 is closed. As Soon as the oil coursing drying chamber 12A hasrecirculated for sufcient time to bring the drying chamber 12A todesired operating temperature, as indicated by temperature gage 43A, thedrying chamber is ready to receive veneer sheet 100.

In oil recovery chamber 12B the characteristic superheated steamenvironment at reduced pressure for recovering residual drying oil fromthe veneer is maintained by controlled delivery of steam from steamsupply 82' through valve 28 into a series of spray heads 152 arrangedabove and below the veneer 100, the vacuumized condition beingmaintained by vacuum pump 154 pulling through recovery condenser 40B andvapor outliow line 32B, the condensate outflow from vacuum pump 154being returned to separator 56 through return line 156. Temperature gage43B reflects the inflow steam temperature, temperature gage 158 reflectsthe ambient temperature of chamber 12B, and vacuum gage 80 monitors thepartial pressure condition of chamber 12B.

Start-up heat for chamber 12B, as well as maintenance heat if necessary,can be appropriately provided, such as by steam heater means or thelike, as indicated at 160.

As Will be apparent, wide variations are possible with respect to thedesign layout of a continuous wood strip processing system such as shownat FIG. 3. By way of typical further example, the various guide rollers102, 104, 112, 114, 120, 122, 124, 126 can be arranged to providemultiple, vertically stacked, wood strip liow arrangements in chambers12A and 12B, to increase throughput capacity of the system and permitfaster infeed from a peeler lathe or the like, with the various flowpaths for the strip being fed in sequence in a manner analogous to thebanks of flow paths conventionally provided in certain veneer drierconveyor arrangements.

In general it will be seen that the wood drying and drying oil recoverytechnique of the invention are applicable to various species of wood(eg. softwood or hard- Wood), to various wood types (eg. heartwood andsapwood) and to various wood forms (e.g. veneer and 1 sawn lumber).However, it is considered that its techniques have especial advantagesand utility in commercial drying of thin, cut grain, pervious Wood formswherein the initial moisture can vaporize and undergo surface diffusionto reach a moisture content equilibrium with the superheated steamenvironment relatively rapidly.

As used herein, the terms oil and drying oil relate to water-immiscibleliquid treating media having a boiling point or distillation rangesubstantially higher than the boiling point of water, and having asubstantial vapor pressure at temperatures in the range of 250-300 F.Various suitable drying oils are known in the art, such as various highboiling hydrocarbon fractions, vegetable oils and mineral oils. Onesatisfactory oil for the purpose is aliphatic (high-aniline) petroleumsolvent Standard Odorless Thinner, marketed by Standard Oil Company ofCalifornia, with a distillation range of 347 F.- 399 F. Additives insmall amounts can also be employed with the drying oil for variouspurposes, such as high-melting point waxes or other water resistantagents, tire-retardant chemicals, preservatives, insecticides, and dyes,for example.

From the foregoing, the various above-indicated objects, advantages andfeatures of the invention, as well as further variations in the detailsof the systems and processes disclosed, will -be apparent to thoseskilled in the art, within the scope of the following claims.

What is claimed is:

1. The process of drying Wood comprising immersing the Wood in a hotbath of water-immiscible drying oil having a boiling point substantiallyabove the boiling point of water under controlled conditions of bathcirculation, temperature and pressure and for a time such that themoisture content of the wood is rapidly reduced to and substantiallystabilized at a selected value in the range of about 2% to about 7% byweight, separating the wood from the oil bath, and maintaining themoisture content of the wood within said range while removing at leastmost of the residual drying oil from the wood by maintaining the Woodunder a vacuum pressure of not more than about one-half atmosphere andat a temperature of at least about '240 F. in a circulating atmosphereof superheated steam until the drying oil content of the wood is lessthan about 0.5% by weight.

2. The process of drying a sheet of peeled veneer during conveyancethereof, comprising a drying phase involving conveying the sheet througha circulating bath of water-immiscible oil heated to an averagetemperature in the range of about 250 F.-300 F., the bath circulationaround the sheet progressively gradually decreasing during its course oftravel through the bath, and the rate of travel of the sheet through thebath being such that a substantially uniform moisture content of fromabout 2% to about 7% is established in the sheet by the time it leavesthe bath, such process further comprising an oil recovery phaseinvolving conveying the oil dried sheet through an environment ofsuper-heated steam under a vacuum pressure of not more than aboutone-half atmosphere While impinging superheated steam thereon to reducethe residual oil content of the strip to less than about 0.5% by weightwithout overdrying the veneer, and recovering oil from the vapors givenolf during bot-h the drying phase and the oil recovery phase.

3. The process of claim 2, comprising maintaining the -oil recoveryenvironment at a temperature substantially .equal to the averagetemperature of the oil bath.

4. The process of drying cut-grain Wood such as veneer and the like to auniform, low but substantial moisture content by oil bath immersion,with oil recovery so that only a negligible amount of residual oilremains in the wood, said process comprising immersing the wood in abath o f waterimmiscible oil under controlled conditions of temperature,pressure and bath circulation so that a superheated steam blanketsurrounds the wood at all times during the immersion thereof in the oil,the

moisture content of the various portions of the wood thereby beingrendered substantially uniform and of a value essentially determined bythe equilibrium moisture content characteristic of the wood at thetemperature of the superheated steam blanket, said process furthercornprising removing the Awood from the oil bath, impinging superheatedsteam On the wood while the wood is still hot and until the oil contentthereof is reduced to less than about 0.5% by weight, such superheatedsteam being under a vacuum pressure of less than about one-halfatmosphere and being at a temperature substantially the same as thetemperature of the oil in which the wood was immersed during dryingthereof, and recovering by condensation essentially all of the oil vaporproduced during drying of the wood and during subatmospheric steamingthereof.

5. The process of recovering water-immiscible drying oil from wood whichhas been dried to a substantially uniform moisture content by immersionin the drying oil at a temperature substantially above the boiling pointof Water; said drying oil recovery process comprising maintaining themoisture content of the wood substantially constant while evaporatingthe residual drying oil from the wood to the point where the final oilcontent is less than about 0.5% by exposing the dried wood while stillhot to an environment of superheated steam under a vacuum pressure ofnot more than about one-half atmosphere.

6. The process of claim 5, wherein said oil is an aliphatic hydrocarbonhaving a distillation range of substantially 350 F. to 400 F.

7. The process of recovering water-immiscible drying oil from thin woodwhich has been dried to a substantially uniform moisture content byimmersion in the oil at a temperature of about 250 4le-300" F.; saidprocess cornprising evaporating the residual drying oil from the wood tothe point where the nal oil content is less than about 0.5% by weight,such evaporation of residual drying oil being effected by exposing thedried wood while still hot to superheated steam under a vacuum pressureof not more than about one-half atmosphere and at a temperature of about250 F.-300 F.

8. The process of recovering water-immiscible drying oil from thin woodwhich has been dried to substantially uniform moisture content byimmersion in oil at a temperature substantially above the boiling pointof water; said process comprising evaporating at least most of theresidual drying oil from the dried Wood by exposure of the dried woodwhile still hot to an environment of superheated steam under a vacuumpressure of not more than about one-half atmosphere for a time to reducethe ,oil content of the wood to less than about 0.5 by weight whilemaintaining the moisture content of the wood substantially constant, inthe range of from about 2% to about 7% by weight.

9. The process of recovering water-immiscible drying oil from Wood whichhas been dried to substantially uniform moisture content by immersion ina bath of high boiling point, water-immiscible oil at a temperaturesubstantially above the boiling point of water and less than the boilingpoint of the oil, said process comprising evaporating most of theresidual drying oil by exposure of the dried wood while still hot to acirculated environment of superheated steam at substantially the sametemperature as the oil bath and under a vacuum pressure of not more thanabout one-half atmosphere for a time to reduce the oil content of thewood to less than about 0.3% by weight.

10. A system for fast drying wood by immersion in hot, Water-immiscibleoil and for recovering residual oil from the wood after immersion; thedrying phase of said system comprising an enclosed drying chambercontaining a hot oil bath, means conveying the Wood into said dryingchamber and through the oil bath, means for heating and recirculatingthe oil bath, and means condensing the vapors given oif in said dryingchamber; and the recovery phase of said system comprising an enclosedoil recovery chamber, means conveying the wood through and out of saidoil recovery chamber, means maintaining said oil recovery chamber undera vacuum pressure of not more than about one-half atmosphere, means forcondensing oil vapor drawn cfr said oil recovery chamber, and meansreturning the oil condensate recovered from said drying chamber and saidoil recovery chamber to the oil bath in said drying chamber.

References Cited by the Examiner UNITED STATES PATENTS Stienen 34-76Payne 34-12 Bachmann 34-76 Willis 34--9.5 Hatch 34-37 McDonald 34-9.5Hutchinson 34-9.5

NORMAN YUDKOFF, Primary Examiner.

1. THE PROCESS OF DRYING WOOD COMPRISING IMMERSING THE WOOD IN A HOTBATH OF WATER-IMMISCIBLE DRYING OIL HAVING A BOILING POINT SUBSTANTIALLYABOVE THE BOILING POINT OF WATER UNDER CONTROLLED CONDITIONS OF BATHCIRCULATION, TEMPERATURE AND PRESSURE AND FOR A TIME SUCH THAT THEMOISTURE CONTENE OF THE WOOD IS RAPIDLY REDUCED TO AND SUBSTANTIALLYSTABILIZED AT A SELECTED VALUE IN THE RANGE OF ABOUT 2% TO ABOUT 7% BYWEIGHT, SEPARATING THE WOOD FROM THE OIL BATH, AND MAINTAINING THEMOISTURE CONTENT OF THE WOOD WITHIN SAID RANGE WHLE REMOVING AT LEASTMOST OF THE RESIDUAL DRYING OIL FROM THE WOOD BY MAINTAINING THE WOODUNDER A VACUUM PRESSURE OF NOT MORE THAN ABOUT ONE-HALF ATMOSPHERE ANDAT A TEMPERATURE OF AT LEAST ABOUT 240*F. IN A CIRCULATING ATMOSPHERE OFSUPERHEATED STEAMUNTIL THE DRYING OIL CONTENT OF THE WOOD IS LESS THANABOUT 0.5% BY WEIGHT.